1. FIELD OF THE INVENTION
The instant invention relates to an improved method and apparatus for the ultrasonic inspection of pipe and tanks. More particular, the invention relates to a couplantless apparatus used for the external, longitudinal detection of defects such as cracks and thin spots in the circumferential belts of pipe or tank walls without the need to encircle the pipe or tank with an array of transducers. The instant invention further relates to a method of operation whereby a computer Controlled Electromagnetic Acoustic Transducer (EMAT) system is utilized for transmitting and receiving ultrasonic beams induced through a tank wall or circumferentially around a pipe wall while traveling longitudinally along the length of the pipe or tank to detect thin spots without the use of a liquid couplant.
2. GENERAL BACKGROUND
One of the most widely used techniques for non-destructive evaluation of tubular members, such as steel pipe, involves the use of sonic beams of ultrasonic frequencies. Typically, the apparatus employed in such techniques includes a piezoelectric crystal which produces ultrasonic vibrations in response to the application of a regulated voltage. When inspecting a tubular member or pipe for flaws or wall thinning, such techniques routinely rely on a method whereby such an apparatus, called a transducer, is maintained in a position relative to the pipe surface and produces ultrasonic waves for coupling through a flowing column of liquid to the pipe. In order to overcome the limitations of a large fixed array of transducers, surrounding the pipe with a couplant or, the inability in certain field situations, to maneuver the pipe into such an array, it was theorized in the prior art that better results could be obtained through the use of an electromagnetic acoustic transducer (EMAT) which would eliminate the use of a liquid couplant, and the need of the apparatus to encircle the pipe. Thus, EMATS have been found effective in launching and detecting ultrasonic waves across a small air gap by the electromagnetic induction process and can operate on rough and unprepared surfaces.
Various means have been employed utilizing EMAT'S for inducing detectable waves into metal for pipe inspection purposes. U.S. Pat. No. 4,092,868 discloses a means for inducing Lamb Waves electromagnetically into a metal object to be inspected. Such waves fill the entire cross section of the material rather than a narrow beam of ultrasonic energy. However, the device as disclosed is incapable of focusing such ultrasonic energy in the object under inspection. Although Thompson in U.S. Pat. No. 4,092,868 teaches a method of internal pipe inspection utilizing ultrasonic transducers in cooperation with Lamb Waves to detect defects in a pipe line, it still relies on two separate receiver coils located within north-south magnets, located at a predetermined circumferential distance apart. Therefore, defects can only be detected between the two magnets and their receivers.
U.S. Pat. No. 4,691,572 issued to van den Berg et al. discloses the use of ultrasonic electromagnetic transducers for the internal inspection of pipelines which requires no acoustic compliant between the transducer and the tube wall. Van den Berg et al. further teaches the applicability of ultrasonic, electromagnetic transducers in an apparatus used for internal pipe inspection which is not restricted to the use of Lamb Waves but is also effective when elastic shear waves are generated in the tubular walls. It is here that the similarity between the present invention and that of van den Berg et al. ends. Van den Berg teaches away from the present invention by advocating the detection of the ultrasonic waves, by a receiver coil located around the south pole of the permanent magnet, after being reflected against the exterior surface of the pipe wall. He also indicates that the south pole should be in near proximity to the pipe surface. These and other distinctions will become more apparent in the following disclosure. The related prior art does not suggest any means of electrically driving or controlling the scanner along the pipe being inspected. Nor does the prior art disclose an external method of inspecting a pipe line for defects when the pipe is charged.
The design of electromagnetic acoustic transducers and their application in pipeline inspection remains a somewhat empirical art, and a great many promising constructions have been proposed, tried and abandoned. As a consequence of the above noted development of the art, the requirements for a low cost, low noise, mechanically simple, and inexpensive apparatus and procedure is apparent. It is to this apparent need that the present invention addresses itself.